Field of the Invention
The present invention relates to a phase difference compensating element that is formed with an inorganic material and has a high heat resistance. More particularly, the present invention relates to a phase difference compensating element and a projection-type image projecting device that utilize an in-plane phase difference with a birefringent layer, and a phase difference in obliquely-incident light with a dielectric multi-layer film, with respect to light in a used wavelength band. This application claims benefit of priority to Japanese Patent Application No. 2013-81775, filed on Apr. 10, 2013, and Japanese Patent Application No. 2014-80245, filed on Apr. 9, 2014, the entire contents of which are incorporated by reference herein.
Description of Related Art
Conventionally, a phase difference compensating element is formed with inorganic optical single crystal such as crystallized quartz, or a stretched polymer film. Inorganic optical single crystal as a phase difference compensating element has excellent durability and reliability, but is costly in raw materials and processing. Inorganic optical single crystal also has a problem of relatively high angle dependence with respect to incident light. Meanwhile, a stretched polymer film is the most widely-used phase difference compensating element, but easily deteriorates with heat and UV rays. Therefore, a stretched polymer film has a problem in durability.
Also, an obliquely-deposited film having an oblique columnar structure (an obliquely-deposited phase difference element) is known as a phase difference compensating element. In this obliquely-deposited film, an arbitrary phase difference can be set by adjusting the thickness, in principle, and a larger area can be easily obtained. Also, lower costs can be realized through mass production. As an inorganic material is used, a phase difference compensating element with a high light resistance and a high heat resistance can be provided.
Also, so as to improve contrast characteristics and viewing angle characteristics, optical compensation techniques using phase difference compensating elements are being used in projection-type image projecting devices these days. An example of an optical compensation technique using a phase difference compensating element is black luminance correction in vertically-oriented liquid crystal shown in FIG. 29.
In vertically-oriented liquid crystal 100, liquid crystal molecules are vertically-oriented in a no-voltage applied state (black state). When light flux vertically enters a reflective light modulating element 110 including this vertically-oriented liquid crystal 100, any birefringence is not generated. Therefore, light flux that has entered a reflective polarizer 120 and been turned into predetermined linearly-polarized light again enters and passes through the reflective polarizer, without any polarization disturbance. Thus, no light leaks into the screen.
However, with respect to light that has entered the reflective light modulating element 110 with a predetermined angle, a birefringence is generated. Therefore, light flux that has entered the reflective light modulating element 110 is transformed from linearly-polarized light to elliptically-polarized light. As a result, part of the light that has reentered the reflective polarizer 120 reaches the screen, resulting in poorer contrast.
Meanwhile, so as to restrain orientation disturbance among liquid crystal molecules due to a transverse electric field, and to improve the response speed of liquid crystal molecules, there is a suggested technique by which liquid crystal molecules are tilted at a predetermined angle (pretilt angle) with respect to the plane of the reflective light modulating element. In this case, however, the polarization state of light flux that has vertically entered the reflective light modulating element is also disturbed by a birefringence, resulting in poorer contrast.
Various techniques have been suggested as methods for compensating the above described polarization disturbance and realizing an optimum polarization state. Examples of such suggested methods include a method of conducting phase difference compensation by providing a phase difference compensating element such as the above described crystallized quartz in a position parallel to the surface of a reflective light modulating element (see Patent Literature 1, for example), and a method of conducting phase difference compensation by providing an organic material having a birefringence, such as a polymer film, in a position parallel to the surface of a reflective light modulating element (see Patent Literatures 2 and 3, for example).
However, in a case where a method of processing single crystal serving as an optical compensation element is employed, particularly when compensation is to be conducted by taking into account even the pretilt angle of liquid crystals, cutting needs to be performed at a predetermined angle with respect to the crystal axis, and extremely high accuracy is required in material cutting and polishing, leading to high costs.
Also, where compensation is to be conducted with a polymer-type stretched film taking into account even the pretilt angle of liquid crystal molecules, there is the need to prepare a biaxial phase difference film or a combination of phase difference films. By this method, manufacturing is relatively easy, but deterioration due to heat or UV rays easily occurs as described above. Therefore, there is a problem in durability.
Meanwhile, Patent Literature 4 discloses a phase difference compensating element using formation of thin films of dielectric materials. This phase difference compensating element includes a negative C-plate formed with alternately-stacked high and low refractive index materials, and an O-plate formed with two or more obliquely-deposited films. This phase difference compensating element corrects polarization disturbance in light obliquely-incident on the reflective light modulating element with the use of the negative C-plate having a structural birefringence generated by the alternately-stacked high and low refractive index materials, and corrects polarization disturbance caused by the pretilt angle with the use of the O-plate formed with two or more obliquely-deposited films.
However, in the formation of the O-plate through oblique deposition, the deposition angle needs to be set within a certain range so as to generate a birefringence, and the column growth angle is restricted within a predetermined width. Oblique particles grown in this manner are not necessarily suitable for correcting polarization disturbance caused by the pretilt angle. There is also a disclosure that a total of 80 stacked layers are necessary in manufacturing the negative C-plate, and therefore, higher costs and a longer lead time are feared.
Patent Literature 1: JP 2005-172984 A
Patent Literature 2: JP 2007-101764 A
Patent Literature 3: WO 2009/001799 A
Patent Literature 4: JP 2006-171327 A